1. Field of the Invention
The invention relates generally to the field of marine geophysical surveying. More particularly, the invention relates to methods for controlling the spatial distribution or geometry of an array of geophysical sensor streamers towed behind a survey vessel.
2. Background Art
Marine geophysical surveying systems such as seismic acquisition systems and electromagnetic survey systems are used to acquire geophysical data from formations disposed below the bottom of a body of water, such as a lake or the ocean. Marine seismic surveying systems, for example, typically include a seismic survey vessel having onboard navigation, seismic energy source control, and geophysical data recording equipment. The seismic survey vessel is typically configured to tow one or more (typically a plurality) of laterally spaced apart sensor streamers through the water. At selected times, the seismic energy source control equipment causes one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals generated by various sensors on the one or more streamers in response to detected seismic energy are ultimately conducted to the recording equipment. A record with respect to time is made in the recording system of the signals generated by each sensor (or groups of such sensors). The recorded signals are later interpreted to infer the structure and composition of the formations below the bottom of the body of water. Corresponding components for inducing electromagnetic fields and detecting electromagnetic phenomena originating in the subsurface in response to such imparted fields may be used in marine electromagnetic geophysical survey systems.
The one or more sensor streamers are in the most general sense long cables that have geophysical sensors disposed at spaced apart positions along the length of the cables. A typical streamer can extend behind the geophysical survey vessel for several kilometers. Because of the great length of the typical streamer, the streamer may not travel entirely in a straight line behind the survey vessel at every point along its length due to interaction of the streamer with the water and currents in the water, among other factors.
Streamers towed by a vessel configured for towing multiple streamers are associated with equipment that maintain the forward ends of the streamers at selected lateral distances from each other and from the centerline of the survey vessel as they are towed through the water. Such multiple streamer systems are used in what are known as three dimensional and four dimensional geophysical surveys. A four dimensional seismic survey is a three dimensional survey over a same area of the Earth's subsurface repeated at selected times. The individual streamers in such systems are affected by the same forces that affect a single streamer.
The quality of geophysical images of the Earth's subsurface produced from three dimensional surveys is affected by how well the positions of the individual sensors on the streamers are controlled. The quality of images generated from the detected signals also depends to an extent on the relative positions of the sensors being maintained throughout the geophysical survey. Various devices are known in the art for positioning streamers laterally and/or at a selected depth below the water surface. U.S. Pat. No. 5,443,027 issued to Owsley et al., for example, describes a lateral force device for displacing a towed underwater acoustic cable that provides displacement in the horizontal and vertical directions. The device has a hollow spool and a rotationally mounted winged fuselage. The hollow spool is mounted on a cable with cable elements passing therethrough. The winged fuselage is made with the top half relatively positively buoyant and the bottom half relatively negatively buoyant. The winged fuselage is mounted about the hollow spool with clearance to allow rotation of the winged fuselage. The difference in buoyancy between the upper and lower fuselage maintains the device in the correct operating position. Wings on the fuselage are angled to provide lift in the desired direction as the winged fuselage is towed through the water. The device disclosed in the Owsley et al. patent provides no active control of direction or depth of the streamer, however.
U.S. Pat. No. 6,011,752 issued to Ambs et al. describes a seismic streamer position control module having a body with a first end and a second end and a bore therethrough from the first end to the second end for receiving a seismic streamer therethrough. The module has at least one control surface, and at least one recess in which is initially disposed the at least one control surface. The at least one control surface is movably connected to the body for movement from and into the at least one recess and for movement, when extended from the body, for attitude adjustment. Generally, the device described in the Ambs et al. patent is somewhat larger diameter, even when closed, than the streamer to which it is affixed, and such diameter may become an issue when deploying and retrieving streamers from the water.
U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes a method for controlling the navigation of a towed seismic streamer using “birds” affixable to the exterior of the streamer. The birds are equipped with variable-incidence wings and are rotatably fixed onto the streamer. Through a differential action, the wings allow the birds to be turned about the longitudinal axis of the streamer so that a hydrodynamic force oriented in any given direction about the longitudinal axis of the streamer is obtained. Power and control signals are transmitted between the streamer and the bird by rotary transformers. The bird is fixed to the streamer by a bore closed by a cover. The bird can be detached automatically as the streamer is raised so that the streamer can be wound freely onto a drum. The disclosed method purportedly allows the full control of the deformation, immersion and heading of the streamer.
UK Patent 2,364,388, by Canter et al. discloses a method of repeating a marine seismic survey of a sub-surface area that has been previously surveyed.
U.S. Pat. Nos. 6,932,017; 7,080,607; 7,222,579 and 7,293,520 (all based directly or indirectly on PCT Application PCT/IB99/01590) describe various aspects of a control system for positioning of marine seismic streamers.